This invention relates in general to the purification of water and more particularly to a method and apparatus for producing high purity, laboratory quality water.
In laboratory work, water having various levels of purity is required for different laboratory projects. The highest quality water is referred to as "type I" water by several professional organizations and approaches the theoretical maximum level of purity (approximately 18 megohms). Unless water having this extremely high level of purity is needed, less pure water is used.
Purified water is normally provided by reverse osmosis (RO) treatment which makes use of a thin membrane to produce product water that is 95% salt free. The reject water from the membrane contains 95% or more of the salts and is usually discarded. The product water from the RO cartridge is typically stored in a storage vessel from which it is drawn when needed. Polishing cartridges which remove dissolved contaminants and colloidal particles can be used to further process the product water and provide extremely high purity type I water which is likewise stored in a tank so that it is available when needed.
The storage of purified water and particularly type I water is undesirable because contaminants from the storage vessel tend to leach into the water and degrade its purity. In addition, stagnant water in the storage vessel is subject to bacteria contamination which causes further degradation of the purity. Stagnant water in drains and other parts of the plumbing system can cause similar problems. Another problem is that the RO membrane tends to become clogged with inorganic scale and bacteria which reduce its effectiveness and useful life. The membranes are high cost items, and the need to frequently replace them adds appreciably to the cost of producing high quality water.